We recently discovered discrete subnuclei marked by cells are calbindin-D28K (CaBP)-positive within the suprachiasmatic nucleus (SCN; Silver et al., 1996). These cells receive direct photic input, a feature expected on cells on the input pathway to pacemakers, or directly within pacemaker cells. Much to our surprise, discrete lesions of this region resulted in loss of locomotor activity rhythmicity, even though such lesions spared other SCN compartments (marked by VIP- and VP-positive cells). Conversely, animals with lesions that spared the CaBP subregion sustained circadian locomotor rhythms. These results indicate that the pacemaker cells controlling locomotor rhythmicity are localized to the region of CaBP-positive cells. The results challenge a long held assumption - namely that any part of the SCN is capable of sustaining circadian rhythmicity. This notion derives largely from lesion studies, in which circadian rhythmicity is sustained following partial destruction of the SCN, which results in survival of about 25% of the nucleus. Restated, previous studies did not provide evidence for a critical area of the SCN necessary for rhythmicity, even though in individual animals, one or another response was lost following lesions. We now propose to examine three specific issues: 1) whether cells in the CaBP subnucleus are necessary for circadian rhythms in other responses other than locomotor- associated rhythms (or whether other rhythms are controlled by different SCN pacemakers), 2) the afferent and efferent connections of cells in the CaBP subnucleus, and 3) whether cells of this subnucleus express properties (free-running, entrainment, phase-shifting) expected of pacemakers. This work is poised to examine fundamental questions about the organization of the SCN: Are all SCN cells pacemakers controlling rhythmicity in other brain regions? Are some cells oscillators, which in turn are driven by SCN pacemakers (or, are all cells pacemakers)? Do different subsets of SCN cells regulate different rhythms (reach different targets)? Or, is there a core of cells that regulates all circadian rhythms?